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Re: [Xen-devel] [PATCH] xen/arm64: Don't zero BSS when booting using EFI

Hi Jan,

On 03/02/17 07:41, Jan Beulich wrote:
On 02.02.17 at 20:25, <julien.grall@xxxxxxx> wrote:
@@ -261,6 +265,8 @@ GLOBAL(init_secondary)
         sub   x20, x19, x0           /* x20 := phys-offset */

         mov   x22, #1                /* x22 := is_secondary_cpu */
+        /* Skip zero BSS on secondary CPUs to avoid nasty surprises. */
+        mov   x26, #1                /* X26 := skip_zero_bss */

         mov   x24, #0                /* x24 := CPU ID. Initialy zero until we
@@ -314,8 +320,8 @@ common_start:

 el2:    PRINT("- Xen starting at EL2 -\r\n")

-        /* Zero BSS On the boot CPU to avoid nasty surprises */
-        cbnz  x22, skip_bss
+        /* Zero BSS only when requested to avoid nasty surprises. */
+        cbnz  x26, skip_bss

Comparing the original comment here with both this and the
earlier hunk, I think the intended meaning is lost. Zeroing the
BSS on secondary CPUs is certainly a bug, not a nasty surprise.
What I think the original comment is meaning to say is "the
BSS should have been zeroed already, but let's better not rely
on that".

This is not the original meaning. On non-EFI setup BSS will not be zeroed before hand as the loader of Xen does not know the size of BSS.

> Another aspect is whether this original meaning of
the comment actually holds on ARM, since the Xen image is not
an ELF binary (only xen.axf is).


Julien Grall

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